In recent years, with increase in the production of portable and cordless equipment, demands for small and light non-aqueous electrolyte secondary cells having a high energy density have increased, and development for non-aqueous electrolyte secondary cells having excellent characteristics have been desired much more than before.
As a positive electrode material for non-aqueous electrolyte secondary cells, LiCoO2, LiNiO2, LiMn2O4 and the like are used, and particularly, a large quantity of LiCoO2 is used from the aspect of safety, capacity and the like. In this material, since lithium in the crystal lattice escapes into the electrolyte as lithium ions when charged, and the lithium ions are inserted into the crystal lattice from the electrolyte when discharged, the material manifests the function as the positive electrode active material.
Theoretically, one lithium atom can be released from or inserted into one LiCoO2 lattice. However, if the majority of lithium are released or inserted, LiCoO2 is violently deteriorated, and especially cycle properties are significantly damaged. Therefore, in the present state, only about 0.55 lithium is released from or inserted into one LiCoO2, and a capacity of only about 150 mAh is used for 1 g of LiCoO2.
Although increase in the capacity is expected by releasing and inserting a larger quantity of lithium atoms, if lithium are released or inserted in present quantities or more, the violent deterioration of LiCoO2 occurs due to the phase transition of the LiCoO2 crystal lattice, accompanying damage of particles and the crystal lattice, and the elution of cobalt ions from the crystal lattice, causing a problem of difficulty to secure satisfactory cycle properties.
Although there are approaches to improve the cycle durability at 4.5 V by doping 5% by weight of zirconium into LiCoO2, the initial capacity lowers significantly, and cycle durability is also unsatisfactory (refer to Z. Chen and J. R. Dahn, 11th International Meeting of Lithium Battery, Jun. 23-28, 2002, Monterey, USA, Abstract No. 266).
Therefore, the object of the present invention is to provide a positive electrode active material for a high-capacity and highly safe lithium ion secondary cell for high voltage that excels in prevention of deterioration at high voltage, and excels in cycle durability.